Date-Time dangling unit fix

Hi all,

Attached is a patch to fix a parsing error for date-time types that
allow dangling units in the input. For example,
`date '1995-08-06 m y d'` was considered a valid date and the dangling
units were ignored.

Intervals also suffer from a similar issue, but the attached patch
doesn't fix that issue. For example,
`interval '1 day second month 6 hours days years ago'` is parsed as a
valid interval with -1 days and -6 hours. I'm hoping to fix that in a
later patch, but it will likely be more complicated than the other
date-time fixes.

- Joe Koshakow